This invention relates generally to the monitoring of a water flow conduit to detect water leakage, and more particularly to a monitoring system in which like flowmeters, each generating a pulse signal whose frequency depends on flow rate, are respectively interposed in the inlet and outlet lines of the conduit to produce output signals which are compared to determine whether a frequency difference exists therebetween indicative of leakage.
Iron ore is reduced to pig iron in a blast furnace. Circumferentially disposed near the top of the hearth section of the furnace are a plurality of tuyeres which function to admit gas or air therein for combustion, each tuyere being cooled by water passing through a flow conduit. It is important to detect water leakage from the conduit; for should leakage occur, the quality of iron produced by the furnace will be degraded. Moreover, with heavy water leakage, there is danger of explosion. Monitoring of the cooling water is therefore essential.
Since iron ore is thrown into the blast furnace equipped with tuyeres, these tuyeres may be damaged thereby, causing leaks in the water cooling conduits. It is therefore of the utmost importance, in order to prevent degradation of the quality of the product or to avoid the danger of explosion, that water leakages be detected in their early stages.
To carry out such monitoring it is known to use a pair of vortex-shedding or Karman-type flowmeters, one being interposed in the inlet line to the cooling conduit, and the other in the outlet line therefrom. A vortex flowmeter includes an obstacle in the flow path which generates fluidic oscillations as a function of flow rate, these oscillations being sensed to produce a pulse signal whose frequency is proportional to flow rate. The pulse output signals from the pair of flowmeters are applied to a comparator which yields an analog output signal that depends on the difference in frequency between the applied signals. When the frequency difference is zero, this indicates the absence of a leak, but when a difference is detected, this signifies that a leak exists.
In a conventional monitoring arrangement, the pulse output signals from the inlet and outlet flowmeters are converted into analog current signals proportional to the pulse frequency by means of frequency-to-current converters, this analog signal being applied to a difference circuit which yields an analog output signal that depends on the difference between the applied analog current signals. This analog output signal is compared in a comparator with a reference voltage, such that when the analog signal which represents the difference between inlet and outlet flows exceeds the reference voltage, an alarm signal is produced to call attention to the existence of a water leak.
In the typical blast furnace installation, there are usually 20 to 40 tuyeres. Hence to monitor water leakage in the flow conduit associated with the large number of tuyeres, 40 to 80 flowmeters are required, two for each conduit. Preferably these flowmeters are of the Karman-vortex type in which an obstacle in the flow tube causes fluidic oscillations to be produced which are sensed by probes to produce an output pulse signal whose frequency is proportional to flow rate. Such meters have several significant advantages among which are low cost and excellent repeatability.
When however, the cooling water contains contaminants such as rubbish or pieces of thread which adhere to the probes of the Karman-vortex flowmeters, a problem arises, for these contaminants will inevitably not adhere in the same way to the probes of both the inlet and outlet flowmeters. Thus one probe may be more contaminated than the other, as a consequence of which flow rate differences will appear in the measured outputs of these flowmeters. The difference signal resulting from such uneven contamination cannot be distinguished or discriminated from the difference signal caused by water leakage.
Thus in a conventional monitoring system in which the instantaneous flow rates in the inlet and outlet lines are compared to produce an alarm signal, false alarms will be produced as a result of contaminants which adhere to the probes of the flowmeters.
Also in the conventional monitoring system which requires frequency-to-current converters, the flow rate difference derived from these current signals includes fluctuations arising from conversion errors or ripple components in the current signals. In this instance since there is a lower limit in the alarm level setting, a false alarm will be produced when the setting level is made lower than the limit level.